Atomic absorption spectroscopy

ABSTRACT

Method and apparatus for atomic absorption spectroscopic analysis, particularly of a solid sample substance, by sputtering the sample to produce an atomic vapor, illuminating the vapor with spectral radiation including the spectrum of an element to be determined and using a photodetector to measure the quantity of resonance radiation emitted by the vapor as a measure of the quantity of the element in the sample, with or without the use of modulation techniques to obtain usable signal to noise ratios in the detector output.

[ Feb. 22, 1972 [54] ATOMIC ABSORPTION SPECTROSCOPY [72] lnventor: AlanWalsh, Brighton, Victoria, Australia [73] Assignee: CommonwealthScientific and Industrial Research Organization, Victoria, Australia 221Filed: May 1,1969

21] Appl.No.: 821,008

OTHER PUBLICATIONS Gatehouse et al.: Spectrochimica Acta, vol. 16, 1960pages 602- 604 Goleb et al.: Analytica Chemica Acta, vol. 28, May 1963pages 457- 466 Sullivan et al.: Spectrochimica Acta, vol. 21, Apr. 1965,pages 7 27- 730 Bowman et al.: Spectrochimica Acta, vol. 22 Feb. 1966,pages 205-210 Neu et al. Messtechnik, July 1968, pages 154- 159.

[30] Applicam Pmmy Dam Primary Examiner-Ronald L. Wibert May 1, 1968Australia ..37l84/68 Assistant Examiner-F. L. Evans I A ttorney-Sughrue, Rothwell, Mion, Zinn & MacPeak [52] US. Cl. ..356/85, 250/435R, 250/833 UV,

356/97 [57] ABSTRACT [5l] lnt.Cl. ..G0l 3/42 Method and apparatus foratomic absorption spectmscopic [58] FieldolSeai-ch ..3l3/209, a1 1 I frd I bsta b sss/ss-sv, 96-98; 250/435 R, 83.3 uv P at Y 3 F e "F n y thesample to produce an atomic vapor, illuminating the vapor with spectralradiation including the spectrum of an element [56] References Cited tobe determined and using a photodetector to measure the UNITED STATESPATENTS quantity of resonance radiation emitted by the vapor as ameasure of the quantity of the element in the sample, with or 3,137,7586/1964 Mason et a]. ..356/95 without the use f modulation techniques toobtain usable lg; x v 2 2 signal to noise ratios in the detector output.ei er v 20 Claims, 4 Drawing Figures I l I I I I 1 I l I l I I I I I I II PAIENIE|1FB22 I972 3 644. 045

sum 2 or 2 LAMP LAMP I I43 14 0 l I 100 I I I I vacuum CHAMBER flZLAMP Ifl LAMP I I PHOTOMULT/PL/ER I 169 I462 I /I54 L1 1 K160 I I g I l I I iI I l i I L-DEMOD. L-DEMOD. DEMOD. DEMOD.

l I I Ji l- ATOMIC ABSORPTION SPECTROSCOPY This invention is concernedwith atomic absorption spectroscopy and seeks to provide a direct andsimple technique whereby a sample can be analyzed without the need forit to be first taken into solution and then sprayed into an atomizingflame. The techniques of the present invention are particularly suitablefor the direct analysis of metals, alloys and other electricallyconducting sample substances.

Basically, the technique of the present invention involves forming anatomic vapor of the substance to be analyzed by means of a dischargewhich causes sputtering of a sample of the substance illuminating thevapor by radiation including the spectrum of an element to bedetermined, and determining the intensity of resonance radiation emittedby the vapor as a measure of the amount of the element present in thesubstance.

It is preferable, in accordance with the'invention, to employ modulationtechniques to distinguish the signal due to resonance radiation fromthose due to self excitation of the sputtering discharge and/or fromthose due to after-glow from the discharge. The signal-to-noise ratiomay be further enhanced if desired by employing broadband longwavelength absorption filters in conjunction with photomultipliers whichhave a peak sensitivity at short wavelengths, since it is in the latterregion of the spectrum that the main resonance lines of many metals lie.

From another aspect, the invention may be said to include apparatus foratomic absorption analysis comprising, a vacuum chamber, mans within thechamber for mounting the sample and for carrying a discharge to causesputtering thereof, an atomic spectral lamp arranged to illuminate vaporgenerated by said sputtering, a photodetector arranged to receiveresonance radiation from the vapor and to produce an electrical outputsignal, and means connected to the photodetector to selectively blockportions of the output signal.

Preferably, the sample to be analyzed is subjected to the usualprocessing cleaning techniques appropriate to electrodes for use inelectric discharge tubes. The vacuum chamber is preferably filled with arare gas at low pressure and arrangements may be made to circulate thegas and clean it. The vacuum chamber may contain an anode electrode sothat the discharge can be struck in said gas between the anode and thesample substance which serves as cathode. In order to avoid detection ofradiation emitted directly from the atomic spectral lamp, thephotodetector should be arranged so as to receive radiation from adifferent direction to that in which the lamp transmits.

According to another optional feature, the sample to be analyzed issecured to a force-cooled projection within the vacuum chamber so thatthe temperature of the sample may be controlled during the analysis;and, for this purpose, temperature sensing means may be attached to theprojection. In order to confine the discharge to the sample, a tubularsleeve of insulating material is preferably placed around the projectionand spaced a small distance therefrom.

In order to further portray the nature of the present invention,particular embodiments thereof will now be described by way of exampleand illustration only. In the following description reference will bemade to the accompanying drawings, in which:

FIG. 1 is a semidiagrammatic elevation of a specimen chamber suitablefor use in the systems of FIGS. 2 and 3,

FIG. 2 is a block diagram of the components of one system embodying thepresent invention and employing simple modulation techniques,

FIG. 3 is a block diagram of a more complex system than that shown inFIG. 2, and

FIG. 4 is a diagram of the layout of a system employing a plurality oflamps for determination of a corresponding number ofelements.

Referring now to FIG. 1 of the drawings in particular, the samplechamber is housed within a lighttight enclosure l2 together with theatomic spectral lamp l4 and the photomultiplier (not shown) which, inthis case, is mounted in the same horizontal plane as the lamp 14 but isdisposed at right angles thereto. Chamber 10 consists of a top plate 16,a cylindrical body 18 and a baseplate 20 which are formed from a metalsuch as stainless steel and rest one upon the other and are sealed in avacuumtight manner by suitable O-rings The body 18 has two pairs ofdiametrically opposed windows 22 sealed in apertures formed in its upperperiphery, the lamp 14 being arranged in line with one pair whilephotomultiplier is arranged in line with the other. If desired, thewindows adjacent the lamp and/or the photomultiplier may be coated orfitted with a broad band long wavelength filter material.

Baseplate 20 mounts: an exhaust pipe 24 through which the chamber 10 maybe evacuated by means of a vacuum pump (not shown) connected thereto; apair of thermometer leads 26 which are sealed therein by means of aninsulating ceramic or glass seal; a sample mount assembly 28; a gasinlet pipe 30; and, an anode electrode wire 32 which is sealed into theplate 20 but insulated therefrom. The sample mount assembly 28 includes:a hollow sample mounting stud 34 (formed of copper, for example) whichis sealed into but insulated from the baseplate 20 so that its lower andopen end is accessible from below the baseplate; a sample button 36which is threaded internally so as to screw onto the stud 34 as shownand incorporates an uppermost recess within which a sample powder orpellet 38 may be pressed; and an insulating cylindrical sleeve 40 whichfits concentrically about stud 34 and rests on baseplate 20. Near theupper threaded portion of the stud 34, a resistance thermometer orthermocouple 42 is wound and its leads are connected to the terminals26. A water cooling jet tube 44 is located concentrically with thehollow base of the stud 34 so as to be able to project a cooling jet ofwater onto the interior walls thereof.

In operation, a sample pellet is pressed into the button 36 by means ofa suitable press located externally to the chamber 10, and the button isscrewed onto stud 34 after the top plate 16 has been removed. The topplate is then replaced and the chamber evacuated via pipe 24 until thedesired pressure has been reached, whereupon, a rare gas such as argonis admitted via pipe 30 to furnish the working pressure and atmosphere.A gaseous discharge is then struck between the sample pellet as cathodeand anode 32, the cathode connection being made to the base of stud 34.After sufficient sputtering of the sample has taken place to thoroughlyclean its surface, the flow of cooling water from pipe 44 is adjusted soas to maintain the sample at a predetermined temperature as indicated bythe resistance thermometer or thermocouple 42. The vapor generated inthe immediate vicinity of the sample pellet is then illuminated byradiation from lap 14 characteristic of the element for which theanalysis is being performed, and the resonance radiation is detected bythe photomultiplier Referring now to FIG. 2, the simplest system-whichis suitable where the element being determined is present within thesample in a few percent-4s that where the sputtering discharge betweenthe sample 38 and the anode 32 is generated by direct current from a DCpower supply 50 and the spectral lamp 14 is supplied from an AC supply52, the output signal from the photomultiplier 54 being amplified by anAC amplifier 56 before being fed to an indicator or output circuit 58.However, even where the concentration of the determined element in thesample is of the order of a few percent, it may be found that theintensity of the pulsating illumination is insufficient to provideadequate discrimination over the emission from the discharge for certainelements. Some improvement can be obtained through operating amplifier56 as a synchronous demodulator controlled in phase with the AC supply52as indicated by the dotted linebut where such improvement is marginal,the system of FIG. 3 is preferred.

In FIG. 3, lamp 14 again irradiates the vapor produced by the dischargebetween anode 32 and sample 38 within chamber 10, and thephotomultiplier 54 may still be directed at the vapor in the same way;but, in this case, the discharge is It) I 024 (N05 pulsatory beingsupplied by an AC power supply 60, and the illuminating spectralradiation is made constant, the lamp being supplied by a DC supply 62.

The timing of the pulses from supply 60 is determined by a timing ortrigger circuit 64 which also controls a synchronous detector-rgating-circuit 66 so that the signal from tube 54 is blocked during eachdischarge pulse and passed to the output circuit 68 only between pulses.

The above described system may be modified by further controlling thecircuit 66 so that the signal is passed only for a brief period(relative to the discharge cycle) following the termination of thedischarge pulse. Preferably, this gated period is delayed somewhat untilthe afterglow of the vapor has died down sufficiently to give optimumsignal to noise ratios. Thus, the signal from the vapor may be detectedfor a short period during which it remains substantially constant andwhen the after glow is not examined.

It will be noted from the above description with respect to FIG. 3 that,if desired, the power supply 62 to the lamp 145 may be pulsed insynchronism with the gating of the signal by circuit 66; and this hasbeen found to offer some advantages in terms of signal-to-noise ratio.(Such operation is indicated by the broken line in FIG. 3.) An extensionof this technique envisaged by the invention is one where thecontrolling circuit deliverscontrol pulses at a greater rate to thepower supply 62 and the gate 66 than to the power supply 60 so that thecircuit 66 is rendered ineffective-as before-during the discharge pulseand so that, in addition, it is made to discriminate in favor of asignal at the frequency at which the light source 24 is pulsed.

FIG. 4 illustrates a further embodiment of the present invention whichenables simultaneous or very rapid analysis for a number of elementsfourin the illustration. Numeral 100 indicates a vacuum chamber similar tothe vacuum chamber but provided with five windows (not indicated)enabling entry of radiation from each of four atomic spectral lamps 140to 143 respectively, and exit of resonance radiation to aphotomultiplier 154. The photomultiplier is again of the type havingpeak sensitivity at or near the ultraviolet end of the electromagneticspectrum and the window associated with the photomultiplier is providedwith a broad band or absorption filter which is effective at longwavelengths. Similar filters may be provided on the other windows.

The chamber is again provided with means to sputter a sample at or aboutthe center thereof to produce an atomic vapor, and the lines ofradiation from the lamps 140 to 143 converge upon the region at whichvapor will be produced. Photomultiplier 154 is arranged to face thisregion and is also located so as to be out ofalignment with each of thelamps 140 to 143.

Lamps 140 to 143 are arranged to emit radiation including spectra ofrespective elements to be determined. Ifit is desired to examine asample simultaneously for each of these elements, the apparatus can beprovided with four synchronous demodulators 150 to 153 respectively thedemodulators feeding respective meters indicated in the figure. Suchapparatus is provided with a power supply (not shown) by means of whichthe currents fed to lamps 140-443 are varied, the variation in thesupply current being different for each of the lamps. As indicated bythe dotted line 160 to 163 respectively, the lamps are associated withrespective synchronous demodulators, and the demodulators are controlledto operate in synchronism with their respective lamps. Accordingly, thedemodulators separate the components in the radiation detected by thephotomultiplier 154 corresponding with the radiation emitted by thevarious lamps corresponding indications are given on the various meters.

It it is not desired that a sample be examined simultaneously for allelements, a single demodulator may be used in association with thephotomultiplier 154, and the lamps may be powered by a power supplywhich varies current supply to the lamps at a single frequency. In thiscase however, the lamps must be controlled to operate successively on atime sharing basis so that successive indications are given as to thequantity of each ofthe four elements to be determined.

While particular examples of the application of the principles of thepresent invention have been described, it will be appreciated that manymodifications are possible without departing from the scope of thepresent invention. Similarly, other apparatus and techniques may beemployed which utilize the principles indicated. For example, the mannerin which the sample is prepared and mounted may vary widely. since wirecathodes are both convenient and effective. While it is not thought tobe particularly desirable, the cathode may be made hollow so as to giverise to hollow cathode emission during the discharge pulses and so as toconcentrate the vapor more effectively. Various alternative proceduresand techniques for processing the photodetector signals may be employedfor particular effects or sample substances. As indicated above, thephotomultipliers are preferably of the type having peak sensitivity toradiation at short wavelengths, near the ultraviolet region of thespectrum. The gas inlet 30 and outlet 24 may be connected to a gascirculating system permitting cleaning and reuse ofgas, for instance, ifxenon is to be used. If argon is to be used, however, inlet 30 maysimply be connected to a suitable source of the gas and outlet 24 may beexhausted to atmosphere.

I claim:

1. A method of atomic absorption spectroscopy comprising the steps ofcreating a pulsed discharge between an electrode and a sample of asubstance to be analyzed so that an atomic vapor of the substance isformed by sputtering during the discharge pulses, and a cloud of thevapor is produced under conditions such that if the sample contains anelement to be determined the vapor in the cloud can absorb and reemitresonance radiation characteristic of the element, illuminating thecloud with illuminating radiation comprising a resonance linecharacteristic ofsaid element, determining the intensity of resonanceradiation characteristic of said element reemitted by the cloud inperiods between discharge pulses in a direction such that the reemittedradiation is not mingled with illuminating radiation, and using thedetermined intensity as a measure ofthe concentration ofthe elementpresent in the sample.

2. A method as claimed in claim 1 comprising the steps of modulatingsaid resonance line of the illuminating radiation and determining theintensity of similarly modulated resonance radiation reemitted by thecloud.

3. A method as claimed in claim 2 wherein said resonance line is one ofa plurality of resonance lines in the illuminating radiation, saidplurality of resonance lines being respectively characteristic of aplurality of elements to be determined, the method including the stepsof modulating said resonance lines in respective distinctive manners,and determining the intensity of resonance radiation reemitted by thecloud and modulated in any one of said distinctive manners.

4. Apparatus for use in atomic absorption spectroscopic analysiscomprising a vacuum chamber, means for mounting a sample of a substancewithin the chamber, means for creating a pulsed discharge to causesputtering of the sample so that an atomic vapor of the substance isproduced in the chamber, means for illuminating said vapor withilluminating radiation comprising a resonance line characteristic of anelement to be determined, photodetecting indicator means arranged toreceive resonance radiation reemitted by said vapor in a direction suchthat illuminating radiation is not received and to indicate theconcentration of said element in the sample in response to receivedresonance radiation, and means to render the photodetecting indicatormeans ineffective during each discharge pulse.

5. Apparatus as claimed in claim 4 comprising means to modulate saidresonance line of the illuminating radiation, the photodetectingindicator means being responsive to similarly modulated resonanceradiation reemitted by the vapor.

6. Apparatus as claimed in claim 5, wherein said resonance line is oneof a plurality of resonance lines being respectively characteristic of aplurality of elements and means for modulating said plurality of linesin respective distinctive manners, and the photodetecting indicatormeans being responsive to resonance radiation reemitted by the vapor andmodulated in any of said distinctive manners so that said plurality ofelements can be determined simultaneously.

7. Apparatus as claimed in claim 6 wherein the means for illuminatingthe vapor comprises a plurality of sources arranged to producerespective resonance lines modulated in respective distinctive manners,and the photodetecting indicator means comprises a photodetectorarranged to supply a signal representing received radiation to aplurality of indicator means selects from said signal a componentrepresenting resonance radiation modulated in a similar manner to theresonance line emitted by its respective source.

8 Apparatus for use in atomic absorption spectroscopic analysiscomprising wall means defining a sealable chamber, a projectionextending from the wall means into the chamber and arranged to receive asample of a solid substance at a location spaced from the wall means, anelectrode within said chamber enabling application of an electricalpotential difference between the electrode and the sample so that adischarge can be created between them to cause sputtering of the samplethereby to create in the region of said location a cloud of atomic vaporof the sample under such conditions that if the sample contains aparticular element the cloud will absorb and reemit resonance radiationcharacteristic of that element, a pair of windows in said wall meansfacing each other from opposite sides of said region, an atomic spectrallamp located outside the chamber for producing resonance line radiationcharacteristic of said particular element to illuminate said cloud witha beam of radiation which enters the chamber by way of one window of thepair and leaves by way of the other, and a further window in the wallmeans so located relative to said pair of windows that in use the beamdoes not impinge on it but resonance line radiation absorbed andreemitted by the cloud is permitted to leave the chamber via saidfurther window to be received by photodetector means responsive theretoand located outside the chamber.

9. Apparatus as claimed in claim 8 and further including energizingmeans for generating said electrical potential difference such that inuse a pulsating discharge is produced, and means to render thephotodetector means ineffective during each discharge pulse.

10. Apparatus as claimed in claim 8 and further comprising gas inletmeans and outlet means extending through said wall means to permitforced circulation of rare gas through the chamber in use.

11. Apparatus as claimed in claim 8 and further comprising a sleeve ofinsulating material located around the projection but spaced therefromand operative to localize the discharge on the sample.

12. Apparatus as claimed in claim 8 wherein the photodetector meanscomprises a photomultiplier having peak sensitivity to radiation nearthe ultraviolet region of the spectrum.

13. Apparatus as claimed in claim 8 wherein means is provided to enableforce-cooling of the projection in use.

14. Apparatus as claimed in claim 13 wherein the projection is hollowand extends through the wall means, and said means to enableforce-cooling is located within the hollow projection.

15. Apparatus as claimed in claim 8 wherein said resonance line is oneof a plurality of resonance lines in the illuminating radiation, theplurality of resonance lines being respectively characteristic of aplurality of elements and being modulated in respective distinctivemanners, and the photodetector means being responsive to resonanceradiation reemitted by the vapor and modulated in any of saiddistinctive manners so that said plurality of elements can be determinedsimultaneously.

16. Apparatus for use in atomic absorption spectrophotometric analysisof a sample for a plurality of atomic species comprising means forcreating a cloud of atomic vapor from the sample under such conditionsthat if the sample contains any one of said species the cloud can absorband reemit resonance radiation characteristic of that species a ainst abackground of emissive radiation at wavelengt 5 other than that of theresonance radiation.

means to generate illuminating radiation to illuminate the cloud andcontaining a plurality of resonance lines respectively characteristic ofsaid plurality of species and modulated in respective distinctivemanners,

photodetector means having peak sensitivity in the ultraviolet region ofthe electromagnetic spectrum and arranged to receive, and produce anoutput signal representative of, resonance radiation reemitted by thecloud in a direction such that none of said illuminating radiation isreceived directly,

and demodulator means responsive to the output signal and arranged toselect therefrom components modulated in any of said distinctive mannersand respectively representative of the concentrations in the sample ofthe species represented by the corresponding modulated resonance linesin the illuminating radiation.

17. Apparatus as claimed in claim 16 wherein the means to generate theilluminating radiation comprises a plurality of sources of radiationarranged to generate respective ones of said resonance lines and toilluminate the cloud in a predetermined sequence, the demodulator meansbeing arranged to switch the output signal of the photodetector meansbetween a plurality of output means in a corresponding sequence so thatsaid components are received by respective ones of said output means.

18. Apparatus as claimed in claim 17 wherein the resonance lines areadditionally amplitude modulated at a preset frequency, and thedemodulator means is selectively responsive to that frequency.

19. Apparatus as claimed in claim 16 wherein the means to generate theilluminating radiation is arranged to modulate said resonance lines atrespective distinctive frequencies and the demodulator means comprises aplurality of demodulator output means selectively responsive torespective ones of said frequencies.

20. A method of atomic absorption spectrophotometric analysis of asample for a plurality of atomic species comprising the steps of formingfrom the sample a cloud of atomic vapor under such conditions that ifthe sample contains any one of said species the cloud can absorb andreemit resonance radiation characteristic of that species against abackground of emissive radiation at wavelengths other than that of theresonance radiation, illuminating the cloud with a plurality ofresonance lines respectively characteristic of said species andmodulated in respective distinctive manners, detecting resonanceradiation reemitted by the cloud in a direction such that none of saidilluminating radiation is received directly with photodetector meanshaving peak sensitivity in the ultraviolet region of the electromagneticspectrum and generating an output in response to the detected resonanceradiation, and demodulating the output signal to provide a plurality ofoutput components modulated any of said respective distinctive mannersand respectively representative of the concentrations in the sample ofthe species represented by the correspondingly modulated resonance linesin the illuminating radiation.

1. A method of atomic absorption spectroscopy comprising the steps ofcreating a pulsed discharge between an electrode and a sample of asubstance to be analyzed so that an atomic vapor of the substance isformed by sputtering during the discharge pulses, and a cloud of thevapor is produced under conditions such that if the sample contains anelement to be determined the vapor in the cloud can absorb and reemitresonance radiation characteristic of the element, illumInating thecloud with illuminating radiation comprising a resonance linecharacteristic of said element, determining the intensity of resonanceradiation characteristic of said element reemitted by the cloud inperiods between discharge pulses in a direction such that the reemittedradiation is not mingled with illuminating radiation, and using thedetermined intensity as a measure of the concentration of the elementpresent in the sample.
 2. A method as claimed in claim 1 comprising thesteps of modulating said resonance line of the illuminating radiationand determining the intensity of similarly modulated resonance radiationreemitted by the cloud.
 3. A method as claimed in claim 2 wherein saidresonance line is one of a plurality of resonance lines in theilluminating radiation, said plurality of resonance lines beingrespectively characteristic of a plurality of elements to be determined,the method including the steps of modulating said resonance lines inrespective distinctive manners, and determining the intensity ofresonance radiation reemitted by the cloud and modulated in any one ofsaid distinctive manners.
 4. Apparatus for use in atomic absorptionspectroscopic analysis comprising a vacuum chamber, means for mounting asample of a substance within the chamber, means for creating a pulseddischarge to cause sputtering of the sample so that an atomic vapor ofthe substance is produced in the chamber, means for illuminating saidvapor with illuminating radiation comprising a resonance linecharacteristic of an element to be determined, photodetecting indicatormeans arranged to receive resonance radiation reemitted by said vapor ina direction such that illuminating radiation is not received and toindicate the concentration of said element in the sample in response toreceived resonance radiation, and means to render the photodetectingindicator means ineffective during each discharge pulse.
 5. Apparatus asclaimed in claim 4 comprising means to modulate said resonance line ofthe illuminating radiation, the photodetecting indicator means beingresponsive to similarly modulated resonance radiation reemitted by thevapor.
 6. Apparatus as claimed in claim 5, wherein said resonance lineis one of a plurality of resonance lines being respectivelycharacteristic of a plurality of elements and means for modulating saidplurality of lines in respective distinctive manners, and thephotodetecting indicator means being responsive to resonance radiationreemitted by the vapor and modulated in any of said distinctive mannersso that said plurality of elements can be determined simultaneously. 7.Apparatus as claimed in claim 6 wherein the means for illuminating thevapor comprises a plurality of sources arranged to produce respectiveresonance lines modulated in respective distinctive manners, and thephotodetecting indicator means comprises a photodetector arranged tosupply a signal representing received radiation to a plurality ofindicator means selects from said signal a component representingresonance radiation modulated in a similar manner to the resonance lineemitted by its respective source. 8 Apparatus for use in atomicabsorption spectroscopic analysis comprising wall means defining asealable chamber, a projection extending from the wall means into thechamber and arranged to receive a sample of a solid substance at alocation spaced from the wall means, an electrode within said chamberenabling application of an electrical potential difference between theelectrode and the sample so that a discharge can be created between themto cause sputtering of the sample thereby to create in the region ofsaid location a cloud of atomic vapor of the sample under suchconditions that if the sample contains a particular element the cloudwill absorb and reemit resonance radiation characteristic of thatelement, a pair of windows in said wall means facing each other fromopposite sides of said region, an atomic spectral lamp located outsidethe CHAMBER for producing resonance line radiation characteristic ofsaid particular element to illuminate said cloud with a beam ofradiation which enters the chamber by way of one window of the pair andleaves by way of the other, and a further window in the wall means solocated relative to said pair of windows that in use the beam does notimpinge on it but resonance line radiation absorbed and reemitted by thecloud is permitted to leave the chamber via said further window to bereceived by photodetector means responsive thereto and located outsidethe chamber.
 9. Apparatus as claimed in claim 8 and further includingenergizing means for generating said electrical potential differencesuch that in use a pulsating discharge is produced, and means to renderthe photodetector means ineffective during each discharge pulse. 10.Apparatus as claimed in claim 8 and further comprising gas inlet meansand outlet means extending through said wall means to permit forcedcirculation of rare gas through the chamber in use.
 11. Apparatus asclaimed in claim 8 and further comprising a sleeve of insulatingmaterial located around the projection but spaced therefrom andoperative to localize the discharge on the sample.
 12. Apparatus asclaimed in claim 8 wherein the photodetector means comprises aphotomultiplier having peak sensitivity to radiation near theultraviolet region of the spectrum.
 13. Apparatus as claimed in claim 8wherein means is provided to enable force-cooling of the projection inuse.
 14. Apparatus as claimed in claim 13 wherein the projection ishollow and extends through the wall means, and said means to enableforce-cooling is located within the hollow projection.
 15. Apparatus asclaimed in claim 8 wherein said resonance line is one of a plurality ofresonance lines in the illuminating radiation, the plurality ofresonance lines being respectively characteristic of a plurality ofelements and being modulated in respective distinctive manners, and thephotodetector means being responsive to resonance radiation reemitted bythe vapor and modulated in any of said distinctive manners so that saidplurality of elements can be determined simultaneously.
 16. Apparatusfor use in atomic absorption spectrophotometric analysis of a sample fora plurality of atomic species comprising means for creating a cloud ofatomic vapor from the sample under such conditions that if the samplecontains any one of said species the cloud can absorb and reemitresonance radiation characteristic of that species against a backgroundof emissive radiation at wavelengths other than that of the resonanceradiation, means to generate illuminating radiation to illuminate thecloud and containing a plurality of resonance lines respectivelycharacteristic of said plurality of species and modulated in respectivedistinctive manners, photodetector means having peak sensitivity in theultraviolet region of the electromagnetic spectrum and arranged toreceive, and produce an output signal representative of, resonanceradiation reemitted by the cloud in a direction such that none of saidilluminating radiation is received directly, and demodulator meansresponsive to the output signal and arranged to select therefromcomponents modulated in any of said distinctive manners and respectivelyrepresentative of the concentrations in the sample of the speciesrepresented by the corresponding modulated resonance lines in theilluminating radiation.
 17. Apparatus as claimed in claim 16 wherein themeans to generate the illuminating radiation comprises a plurality ofsources of radiation arranged to generate respective ones of saidresonance lines and to illuminate the cloud in a predetermined sequence,the demodulator means being arranged to switch the output signal of thephotodetector means between a plurality of output means in acorresponding sequence so that said components are received byrespective ones of said output means.
 18. Apparatus as claimed in claim17 whErein the resonance lines are additionally amplitude modulated at apreset frequency, and the demodulator means is selectively responsive tothat frequency.
 19. Apparatus as claimed in claim 16 wherein the meansto generate the illuminating radiation is arranged to modulate saidresonance lines at respective distinctive frequencies and thedemodulator means comprises a plurality of demodulator output meansselectively responsive to respective ones of said frequencies.
 20. Amethod of atomic absorption spectrophotometric analysis of a sample fora plurality of atomic species comprising the steps of forming from thesample a cloud of atomic vapor under such conditions that if the samplecontains any one of said species the cloud can absorb and reemitresonance radiation characteristic of that species against a backgroundof emissive radiation at wavelengths other than that of the resonanceradiation, illuminating the cloud with a plurality of resonance linesrespectively characteristic of said species and modulated in respectivedistinctive manners, detecting resonance radiation reemitted by thecloud in a direction such that none of said illuminating radiation isreceived directly with photodetector means having peak sensitivity inthe ultraviolet region of the electromagnetic spectrum and generating anoutput in response to the detected resonance radiation, and demodulatingthe output signal to provide a plurality of output components modulatedany of said respective distinctive manners and respectivelyrepresentative of the concentrations in the sample of the speciesrepresented by the correspondingly modulated resonance lines in theilluminating radiation.